A new category of equatorial F region plasma irregularities having properties easily distinguised from those of plasma bubbles, depletions, or broadband turbulence has been identified in the data from the retarding potential analyzer (RPA) and the ion drift meter (IDM) on the Atmosphere Explorer satellites AE-C and AE-E. These irregularities are observed in the bottomeside at night in a narrow belt extending approximately ¿12¿ from the dip equator and up to 7500 km or more in the east-west direction. High-resolution data from the RPA reveal nearly sinusoidal waves in the ion number density Ni with peak to peak ΔNi/Ni amplitudes that span more than three orders of magnitude, from values less than 0.1% to more than 100%. These distinctive ''bottomside sinusoidal'' (BSS) irregularities typically have a central wavelength lying in the range from 300 m to 3 km, and relatively narrow spectra. The power spectral index is often large (5 or 6) at the shorter wavelengths, but the spectrum usually decays much less rapidly on the longer wavelength side. The vertical drift velocity also has a wavelike structure that appears to be anticorrelated to within a phase uncertainty of approximately ¿90¿ with the Ni variations, such that regions of lower density plasma move upward and higher density regions move downward with respect to the mean ionospheric drift. Anticorrelated wavelike drift and density fluctuations are consistent with plasma tht is Rayleigh-Taylor unstable. Consideration of altitude localization and cross-field ambipolar diffusion effects can introduce at least a slight maximum in the R-T growth rate near 1 km and also sharplly limit irregularity growth at smaller scale sizes, thus providing a tentative explanation of the peaked Ni spectrum observed. Ionograms from Huancayo display a typical ''mid-latitude'' frequency spread signature when BSS is present.